Grey Area
by KYUUxKYUU
Summary: When Len discovers his sister, Rin, bears a similar soulmate marking to his own, he is forced to decide whether to abide by the laws of destiny or morality.
1. Prologue

**Grey Area : Prologue**

 _Soulmate AU in which one's soulmate bears a matching design to their own in a similar location._

Len distinctively remembered the elation that came with the arrival of his sixteenth birthday—the day each person's soulmate mark appeared. He remembered the excitement of reaching a new milestone in his life and seeing the door of opportunity that had opened before him.

He spent an extra half hour in the bathroom that morning, searching hurriedly for where the mark might be. Finally (with the help of two mirrors) he had located the pinkish spot on the back of his neck, shaped like a lotus.

He was embarrassed at the time, immaturely associating anything pink and flowery with femininity and, when his cheerful parents and wide-eyed little sister wearing her own party hat asked to see his soulmate sign at breakfast, he made up some excuse about it being in an awkward location and changed the subject to when they would eat the vanilla birthday cake.

Looking back now, Len could only shake his head at his younger self's foolishness.

Having a "girly" soulmate mark was hardly worth caring about given everything else that would soon come and strip away everything he thought he knew about the world and himself.

* * *

The next year, his birthday was not as exciting as the previous but he was happy to see his younger sister discovering her own soulmate mark. Their parents has put in just as much effort as last year, preparing a considerably large cake, this time chocolate, as it was the flavor Rin preferred and they had all agreed Rin should choose the cake because it was her most important birthday year.

As expected on a Saturday, she did not wake until midday and did not bother changing out of her pajamas. As she made her way down the stairs their mother greeted her with a question.

"So, what does it look like?"

Everyone understood what the woman was referring to and Rin gave a bright grin in response as she reached the dining room table before turning around and pulling her short, blonde hair up to reveal a mark on her nape.

A pink lotus.


	2. Chapter 1

**Grey Area : Chapter 1**

* * *

He thought he was mistaken—he _prayed_ he had just looked at the tattoo wrong but with each passing moment that he stared at the design, that eerily familiar pink lotus, he knew his eyes were not simply playing tricks on him.

The location, the color, the shade, the petals—every last part matched up perfectly to the one he had hid on his own skin for the past year.

It was the same as his own.

"Wow, that's such a cute design, Rin! You got lucky," Mrs. Kagamine complimented as she examined the marking further.

"Eh, really?"

"Of course. Your father and I have such plain ones."

The room around him was filled with such warm chatter and laughter but Len felt like time had stopped and was shrouded in a cloud of darkness. A thin layer of perspiration coated his pale skin and his throat felt as if he had not seen water in months. His mind was filled the brim with racing thoughts yet none of it was comprehensible except the one truth he did not want to acknowledge.

 _My soulmate is my sister._

The room fell into abrupt silence at the screech of chair legs against hardwood as Len stood up.

"I'm feeling sick. I'm going to my room."

He bolted up the stairs before the family could even think of questioning him further. Following the distinct slamming of a door, the three looked between each other in confusion before Mr. Kagamine shrugged it off as the usual teenage hormones and his wife attempted to regain the light mood with the offer of eggs and toast.

Being alone and left to his own thoughts was not as helpful as he had hoped for it to be but he knew it was better than having to face his family downstairs—especially Rin.

The sister he had lived with since birth. His innocent little sister who had at one time begged him to play "Princess and Knight" with, who had relied on him to protect her from the scary monsters under her bed, and still sometimes came to him for help on particularly difficult algebra equation.

What would she think if she knew?

The law of soulmates had existed for thousands of years, known and practiced by every human on Earth. There had never been a mistake; every person on earth had always had a marking that matched with one other person without exception. It was destiny.

And yet, his soulmate turned out to be his sister. Was there a mistake in the previously solid soulmate system beyond human power?

Not likely…and that was the scariest part.

He had two choices: Ignore fate, keep his own tattoo secret 'til death and pretend he did not know who Rin's soulmate was, no doubt leaving her in a life of anguish thinking she was never able to meet her other half.

Or, he could tell her and love her as a soulmate, breaking one of the biggest social taboos and leading her into a life of being constantly hated and looked down upon with disgusted eyes.

Neither option eased the queasiness in his stomach.

* * *

Monday morning seemed to come faster than usual.

Although his parents gave him strange looks at breakfast, no one questioned his behavior two days ago. He was thankful for that.

On the way to school, Rin had sluggishly complained about the teachers and homework as usual and Len added an occasional comment for politeness' sake. She did not mention the unusual extra decimeters of space between them.

The day had been rather uneventful for the most part until lunch break when Len usually met up with his cross country friends.

As usual, Mikuo had brought some sort of high protein meal that looked more like a train wreck than a sandwich to grow "kick-ass muscles to impress the ladies with" while Kaito devoured a frozen treat from the vending machine and Len enjoyed a _real_ lunch…of bananas.

"It was your sis's birthday a few days ago, right? Did she get her mark yet?"

Ah, the one conversation topic Len wanted so desperately to avoid. He fumbled a bit with the peel of his banana and attempted to answer as calmly as possible before changing subjects.

"Yes, it's a flower or something, I think. Be sure to make it to today's meet on time. Our next race is going to be a lot tougher than Mizuho and you guys have been slacking lately."

Kaito seemed lost in his own world already and Mikuo let out an elongated groan.

"Yeeees, Captain," he replied sarcastically before changing subjects to "the cute transfer student in 2-A", much to Len's relief.

* * *

Afternoon practice ended at 6PM and Len felt exhausted but accomplished as per usual. Rin had already gone home when school ended, as she was not a part of any club, and Len could not help but to feel relieved at that.

After locking up the gym and bidding his last few teammates a goodnight and "work on that starting form", Len headed home.

The weather was a little chilly, as it was an autumn night, but it felt refreshing on his flushed skin.

The peaceful walk was interrupted by the familiar aroma of freshly fried chicken and he could not resist taking a detour to a convenience store on the way.

The air inside the small store was much warmer than outside and brightly lit similarly to a hospital. Locating the chicken and boxing a few into a container was easy enough but, as he walked pass the drinks section, a brown and white row of cartons on the shelf caught his eyes. The small cartons were of Boba—maple milk and balls of tapioca advertised at the "fall flavor".

They were Rin's favorite.

Len was already picking one up before he even really processed it and made his way back to the front of the store to pay.

After a five minute or so wait in the short line, he was at the counter, pulling out his wallet and placing a crisp bill on the money tray.

As the cashier woman reached for the money, he noticed her arm, or rather the designs on it. A tan, minimalist dragon design wrapped around her thin arm from the wrist to the elbow. Although simple, the composition was elegant and seemed very professionally crafted and Len found himself almost mesmerized by its beauty.

"Um, sir…"

Len glanced up at her face after her arm shifted from his view to hide behind her back self-consciously.

"Oh," he said dumbly.

A beat of awkward silence passed before something clicked in his brain to apologize for his rude staring.

"Sorry, I was just admiring your tattoo—it's really beautiful!"

The young woman smiled at this, moving her arm to gaze at it appreciatively herself.

"Thank you…it's my soulmate mark."

The word "soulmate" still made his throat dry and he felt anxious to the leave the store as quickly as possible. His eyes shifted to the nearest exit impatiently.

"To be honest, I didn't like it for a long time."

His blue eyes shifted back to her brown ones at that.

"Even though I was already 24 I still hadn't found my soulmate while all my friends were getting married to theirs and having such an obvious tattoo felt like a constant reminder that I was alone...and then I saw her. She was in the place I least expected it but once we saw each others arms we were so happy. We couldn't stop smiling for days."

She grinned herself as she spoke, almost as if she were watching the moments unfold before her very eyes. Her attention turned back towards him and her smile grew even softer.

"Once you find your own soulmate, never let them go. There is no greater feeling than love that humans can experience…oh! Oops, I almost forgot I was working—here's your change, sir."

Len suddenly didn't feel like eating anymore.

* * *

 _A/N: Thank you for all of the positive and constructive responses I've received within the last few hours. I felt quite nervous about writing this fic after being in "hiatus" for so long. Thank you very much for your support._


	3. Chapter 2

**Grey Area : Chapter 2**

Although the cold winds still nipped at exposed skin mercilessly, everyone seemed to be in good spirits as if it were already spring.

Through the whole walk to school, Rin chattered excitedly about the holiday. She expressed her optimism of the Valentine's Day in a series of "what if I meet my soulmate today" followed immediately by "no way, that's too cliché" and repeat.

It had been a little over a year since Len had discovered his sister's soulmark matched his own and he resolved to keep it a secret from her for the rest of his life. There was no way he would allow his sister to suffer the same emotional trauma he had struggled with for the past thirteen months.

Nowadays, he found it a lot easier to come to terms with and, perhaps he could go as far as to say, he has accepted it. Rather, there are a few benefits he has found. Namely, the "protective older brother" side of him does not have to worry about what kind of potentially dangerous or unworthy person Rin would be fated with while he is off to university next year.

He himself never had a particular interest in romance, unlike Mikuo, to begin with so never having a soulmate does not really bother him.

"What if all this time it's been someone we both know," Rin suddenly asks, looking back and Len with a grin mischievous enough to rival the Cheshire cat.

"Like _Kaito._ "

Len makes a disgusted face and Rin laughs.

* * *

Nothing is out of the ordinary for the first half of school, as teachers keep strict policy on not allowing food or chocolate in class and definitely no PDA outside of break. A few students still pass notes and girls sneak small, handmade packages to their crushes when the teacher turns around.

By lunch, most of the confessions and gifts have been distributed and Mikuo is quick to announce to everyone he sees that the cute new girl class-2A gave _him_ chocolate and no one else.

"Have you even seen her soulmark?" Kaito finally interrupts, Mikuo's excessively bragging beginning to ever wear on his usually calm nerves.

Mikuo sputters vague excuses for a second, at a loss, before answering honestly.

"I can't see it yet—I'll be so disappointed if it's not her!"

Len rolls his eyes at this.

"You say that about _every_ girl you have a crush on. You may as well find out and get it over with before she finds her real soulmate and leaves you in the dust."

Mikuo gasps dramatically with an offended expression.

"At least I'm trying, you haven't even looked for your soulmate!"

"That's the point of soulmates, stupid, you don't have to _look—"_

The bell rings before a full-on argument can start and the three reluctantly make their ways back to class—but not without a few "accidental" pushes and jabs along the way.

By the end of the day even Len received a couple of "Anonymous" cards and treats in his shoe locker. Although he considered himself pretty average in attractiveness and somewhat introverted, he was relatively popular in the school for being the cross-country captain and having one of the highest grades in mathematics and science.

* * *

When Rin discovered that even her _nerdy brother got chocolates_ while she did not get a _single one_ , she proceeded to pout until he stopped to buy her Boba. She was a spoilt brat, he thought. Actually, it was probably his fault that she was like this.

But he did not find himself regretting it at all.

* * *

At home, it was the usual Friday night tradition. Len finished his homework quickly so his weekend would be relatively worry free while Rin opted to play video games, their mother prepared dinner, and their father would be home late from the office.

Over dinner, Rin and their mother discussed Valentine's school gossip and Rin's expectations for her own soulmate while Len zoned out and fiddled with his smartphone.

"So, Len," Mrs. Kagamine suddenly called.

He looked up and felt somewhat cornered and awkward between the two females staring at him so intently.

"Yes?"

"What kind of girl— _or boy—_ do you want your soulmate to be?"

He felt a headache coming on.

"It doesn't matter, right? Regardless of what I think my preferences are, my soulmate is going to be the same person."

Rin rolled her eyes and returned back to her nearly empty plate in disinterest.

"That's no fun. Well, if you _could_ choose your soulmate, who would you want it to be?"

As usual, his mother was annoyingly persistent over the most insignificant of things. He internally groaned before answering.

"I dunno…someone nice I guess? Probably a girl who's happy and cute. It'd be cool if she liked science too."

Rin chokes on her drink in poorly suppressed snickers and their mother opens her mouth to scold the girl but the conversation is broken by the sound of the front door unlocking and Mrs. Kagmine rushes to greet her husband who is home earlier than expected.

When the two are alone at the table Rin playfully mouths the word "loser" and Len makes a ridiculous face in response just to hear her laugh.


	4. Chapter 3

**Grey Area : Chapter 3**

* * *

With the arrival of spring, the school was filled with a mix of optimism and anxiety. The enthusiasm for seniors to finally graduate within a few weeks mixed with the stress of final exams and college planning. Len did not need to worry about tests so much, as his grades had remained consistently high throughout his three years of high school, but he still studied extra for the upcoming weeks, more out of habit than anything else.

As usual, Rin waited until the last minute.

Len pulled his hair into a small, messy ponytail, annoyed with the blonde locks repeatedly obstructing his view. The papers organized by subject across his desk summarize each of the lessons taught in class in the semester so far. Each meticulously color organized and sorted by importance and difficulty. It had already been two hours and the sun was long gone but he had successfully completed only two of the six subjects he'd hoped to review before tomorrow.

'Len,"

Said male looked up from his notes, slightly startled until he recognized the sheepish figure in his doorway. He already had a pretty good idea of what she was there for and he struggled to hold back a smile.

"Yes, Rin?"

"…I need help with math…"

He finally allowed himself to slightly grin as he stood from his chair and pushed his own paper to the side, a silent invitation she wasted no time in accepting. She takes a seat and slides her cluttered notebook over, papers hastily stashed between every few pages with doodles on the sides and countless erase marks on nearly every equation. He leans over her form to read the problem she seems to be stuck on.

"This is a quadratic equation so first you have to set this equal to zero," he begins.

She seems to listen intently, nodding every once in a while when appropriate. Despite her apparent difficulties with mathematics and a few other subjects, Len never once considered his sister "stupid", regardless of how her friends—and ever she herself, referred to her. She was lazy at times, he would admit, but she was talented in a variety of things beyond his own understanding.

When they were younger they would enjoy debating with one another about obscure topics or concepts adults would not question ("if parallel universes exist, is there a parallel universe where parallel universes don't exist?"). They were just fun things to think about the time—much too puzzling for him to ever really go in depth with, but Rin took them much more seriously.

She never failed to throw him off and amaze him with her own philosophical questions and thoughts. She effortlessly played around with concepts and laws of nature he had never even dreamed of. There were times he was convinced his little twelve-year-old sister was a philosophical genius, able to entertain even Aristotle or Socrates with her ideas.

Those days faded rather quickly as they grew up.

For the past several years any attempts to compliment Rin usually ended in being waved off as a "bad joke" regardless of his proclamations of sincerity. Len wishes Rin could see herself as he saw her—as he still sees her, and not just as the "below average" as she had been labeled by her teachers and peers.

"Is the answer 'X equals square root of one'?"

"Not quite—you forgot the negative sign here."

At her confused expression, Len leans forward to point at the part he was referring to with his own pencil.

"Oh," Rin replies simply, quickly erasing her mistakes.

As he watches her recalculate the numbers, his eyes begin to wonder from the scribbles on the page to the pencil then her hand and up, trailing over the rest of her hunched over form. He's suddenly aware of how close she is—his chest mere centimeters from her side. Her shampoo—some kind of citrus, is unreasonably strong all of a sudden and he finds his breath shallow. Her cropped blonde hair, so similar to his own, looks soft in the harsh lightening of his desk lamp. And just below the golden tresses lies a mark on her neck, identical to his own.

The words cross his mind before he can even begin to filter them out.

 _She's so beautiful._

Len involuntarily jumps back in shock at his own thoughts, startling Rin in the process.

"Len…?" she questions.

Len chuckles nervously, feeling a knot forming in his stomach and a sudden dryness in his throat. He purposefully focuses on the math assignment, avoiding eye contact.  
"This is correct—good job. Shall we try another?"

Rin groans loudly.

* * *

The end of exams seemed insignificant after being diluted with all of the other work Len has been doing for the past few days. Fittings for suits, distant relatively constantly calling and arranging visits, college entrance exam preparations, and so on. He does not bother complaining, as his parents seem much busier than himself and they have been spending so much money in preparation for his graduation.

On the other hand, Rin has been bragging about her worry-free summer and all the fun she intends to have playing video games and sleeping in all day. Len made sure to wipe the smirk off her face when he reminded her of the summer homework she'd be sure to get.

Finally, a Friday in the middle of March arrived and a few hundred students were crowded in the school courtyard, clutching flowers, candies, balloons, and various other gifts as they exchanged congratulatory words and farewells with parents and friends. Many were tearing up if not full-out bawling and Len was positive he had accidentally photobombed at least twenty people.

"Len—Len, come take a picture with your running friends!"

Len held back a sigh before obediently approaching his own family and the majority of his cross country team. His own aunts and uncles had drove and flown in from other cities and villages just for his graduation. He personally didn't see the significance of the graduation—he'd have another one in just another few years anyway—but his parents' eyes shone with so much pride, he couldn't disappoint them.

Mikuo and Kaito stood on either sides of him, giving the camera wide grins and peace signs while Mrs. Kagamine counted down from three and snapped a picture with her digital camera. After the picture, most of the members dispersed as they were mostly first and second years who still needed to return to class, but all stopped to wish Len a congratulations and some even gave him gifts.

"Where's Rin," he inquired after noticing her absence from the one of the "family photos".

"I'm not sure, she probably went to say goodbye to her own friends," Mr. Kagamine supplied.

"I think she went off with Miku to go see Gumiya—apparently all the girls are fighting over who gets his second button*," Mikuo said before making some snappy remark about his sister's interest in the "good for nothing green-haired pretty boy".

Kaito replied with something along the lines of "look who's talking", resulting in bickering between the two but Len was not really paying attention to that. The idea of someone—especially someone as well known to be a player as Gumiya—giving his second button to Len's sister irked him more than he expected it would.

The shrieking of a group of girls interrupted his thoughts. Looking towards the noise, he found a cluster of girls—mostly first and second years, huddled around a few third year boys, one of them being Gumiya himself.

It didn't take long to put the scene together and know Gumiya had apparently revealed his soulmark and just given his button to some pretty, blonde second year—who was not Rin, much to Len's satisfaction—that had a matching soulmark and the other girls were barely containing the disappointment and jealousy. Seeing the prized second button was already gone, many of the girls dispersed, including Miku and Rin who both seemed to be pouting and complaining to one another.

"Lily isn't even that pretty yet she's his soulmate—this is so unfair! Miku could be heard whining as she approached her brother and his friends.

"You're just jealous because Gumiya is way out of your league—your soulmate is probably some greasy old guy," Mikuo teased, easily dodging a kick from his sister.

In the distance, Lily giggled and Gumiya placed a haste kiss on her cheek.

Rin remained silent for the remainder of the graduation.

The ride home was filled with excited chatter from the adults, discussing their own high school adventures and raving about the restaurant they were all headed to.

Despite the abundance of noise in the van, Len felt uncomfortable by the silence from his sister as they both looked out of opposite windows in the backseat. Ever since Gumiya, indirectly, rejected her, she seemed to be in a state of depression.

"Um, Rin?" Len finally called, reaching his limit.

Rin blinked slowly before turning to face him, waiting for him to continue.

"About, uh, Gumiya," he began awkwardly, feeling as if even mentioning his name would upset his sister further, "he's not that great you know—I mean, you shouldn't worry about him not being your soulmate; you're way too good for him."

Rin sighed and faced forward. A long silence followed and Len began to think she wouldn't reply at all.

"It's not Gumiya—I'm not really interested, it's just…I wish I found my own soulmate…but it's hard to even imagine someone liking me, y'know?"

Len's lips part in surprise at this.

"Well, _you_ probably don't know," Rin corrected, giving him a look from the corner of her eye, "you're pretty popular with the ladies, playboy. I saw all those girls asking to see your soulmark."

"I am not a—"

"Relax, I'm joking," Rin interrupted, giggling at his expression.

Len shut his mouth at this. Her laughter usually brings him some sort of happiness and peace—but this one felt unusually cold and fake.

 _I wish I found my own soulmate..._

A tsunami of guilt seemed to crash into his body and his blue eyes flickered away to the dark window. His left hand moved to subconsciously rub the ink at the back of his neck. He suddenly wanted to throw up.

 _You really don't want to know_ , he thinks. She'd be horrified if she knew.

He felt unreasonably personally responsible for their fated misfortune. As if, somehow, if he weren't born, this wouldn't have happened. Maybe she'd be paired with someone else who could actually love her as she deserved without it being a forbidden sin.

Without much thought, Len reached for his white uniform top, grasping the second button and pulling it past its limit, straining the white thread until it came off with a dull _pop._ Rin eyed him curiously until he lifted his hand to her face, a single white button in his palm.

"I know it's not the same b-but the whole tradition is stupid anyway so you can just have mine."

Her blue eyes met his own, his cheeks rapidly growing warmer, and time seemed to still for a moment before her lips slowly curved into a soft smile and she gently took the small button from his hand.

"Thank you, Len."

He worried his heart would burst from his chest.

* * *

*There's a tradition of boys giving their uniform's second button to the girl he likes (or just a really cute one, honestly).


End file.
